Re: [gentoo-user] LVM for data drives but not the OS

[BRM]

----- Original Message ----

> From: Joost Roeleveld <joost@××××××××.org>
> On Thursday 07 April 2011 06:52:26 BRM wrote:
> > ----- Original Message  ----
> >
> > > From: Joost Roeleveld <joost@××××××××.org>
> > >
> > > On Thursday 07 April 2011 06:20:55 BRM wrote:
> > > >  ----- Original Message  ----
> > > >
> > > > >  From: Neil Bothwick <neil@××××××××××.uk>
> > > >  >
> > > > > On Thu, 07 Apr 2011 05:22:41 -0500, Dale  wrote:
> > > > > >  I want to do it this  way because  I don't trust LVM enough
> > > > > >  to put   my
> > > > > >
> > > > > > OS
> > > >  > > on.  Just my  personal  opinion on LVM.
> > >  > >
> > > > > This doesn't make sense. Your OS can   be  reinstalled in an hour
> > > > > or two, your photos etc.  are   irreplaceable.
> > > >
> > > > Makes perfect  sense to me as well.
> > > >
> > > > Having installed LVM -  and then removed it due to issues; namely,
> > > > the  fact that  one of the hard drives died taking out the whole LVM
> > > >  group,  leaving the OS unbootable, and not easily fixable. There
> >  > > was a thread on  that (started by me) a while back (over a  year).
> > > >
> > > > So,  perhaps if I had a RAID to  underly so I could mirror drives
> > > > under LVM
> > > >
> > > >  for recovery I'd move to it again. But otherwise it is  just a PITA
> > > >   waiting
> > > >
> > >  > to happen.
> > > >
> > > > Ben
> > >
> >  > Unfortunately, any method  that spreads a filesystem over multiple  disks
> > > can be
> > >
> > > affected if one of   those disks dies unless there is some mechanism in
> > > place that can  handle the  loss of a disk.
> > > For that, RAID (with the exception  of striping, eg. RAID-0)  provides
> > > that.
> > >
> >  > Just out of curiousity, as I never had the need to look  into this,  I
> > > think that, in theory, it should be possible to recover  data  from LVs
> > > that were not
> > >
> > > using  the failed drive. Is this assumption correct or  wrong?
> >
> > If  you have the LV configuration information, then yes. Since I managed to
> >  find the configuration information, I was able to remove the affected  PVs
> > from the VG, and get it back up.
> > I might still have it  running, but I'll back it out on the next rebuild -
or
> > if I have a drive  large enough to do so with in the future. I was wanting
> > to use LVM as a  bit of a software RAID, but never quite got
> > that far in the  configuration before it failed. It does do a good job at
> > what it's  designed for, but I would not trust the OS to it either since the
> > LVM  configuration is very important to keep around.
> >
> > If not, good  luck as far as I can tell.
> >
> > Ben
>
> LVM isn't actually RAID.  Not in the sense that one gets redundancy. If you
> consider it to be a  flexible partitioning method, that can span multiple
>disks,
>
> then yes.
> But  when spanning multiple disks, it will simply act like JBOD or RAID0.
> Neither  protects someone from a single disk failure.
>
> On critical systems, I tend  to use:
> DISK <-> RAID <-> LVM <-> Filesystem
>
> The  disks are as reliable as Google says they are. They fail or they don't.
> RAID  protects against single disk-failure
> LVM makes the partitioning  flexible
> Filesystems are picked depending on what I use the partition  for
>

The attraction to LVM for me was that from what I could tell it supported and 
implemented a software-RAID
so that I could help protect from disk-failure. I never got around to 
configuring that side of it, but that was my goal.
Or are you saying I was misunderstanding and LVM _does not_ contain 
software-RAID support?

Ben